The present invention generally relates to apparatus and methods for voltage regulation in electric power generators and, more specifically, to apparatus and methods of preventing over-voltage in electric power generators during load removal.
Electric power generators are used to provide onboard power to electrical devices in many applications. Two examples of electric power generators are constant speed, constant frequency generators, as shown in FIG. 1-A and variable speed, constant frequency generators, as shown in FIG. 1-B. In such electric power generators it is important to maintain the supplied voltage level within an acceptable range. To accomplish this, as shown in FIGS. 1-A and 1-B, a voltage regulator 10 senses the voltage at a Point of Regulation and sends a signal to a field discharge circuit 102, which controls the current in the field winding 100. By changing the current in the field winding 100, the generator (G in FIGS. 1-A and 1-B) output is controlled as well as the voltage at the Point Of Regulation (POR) 103. In certain situations, such as during the sudden removal of a large load or post certain fault conditions, the generator terminal voltage may reach a high value, which can damage electrical devices connected to the generator. A voltage regulator can be used to reduce the field current in the generator to reduce the terminal voltage. To accomplish this, since the field power is unidirectional, the field current must be dissipated in the field resistance. However, the field resistance is small so the recovery time to return to the desired regulated voltage will be large. This can result in an over-voltage condition on the generator terminal for an unacceptably long period of time resulting in damage to electrical devices receiving power from the generator.
FIG. 2 depicts a block diagram of a conventional generator voltage regulator, such as the voltage regulator 10 shown in FIGS. 1-A and 1-B. The generator voltage regulator 10 includes a constant DC voltage source, shown as +VDC and −VDC. A voltage regulator controller (VRC) 12 receives a signal VPOR from a Point of Regulation, typically an AC generator terminal (not shown). The output of the voltage regulator controller 12 is connected to a pulse width modulation (PWM) controller 14, which in turn has its output connected to the gate of a transistor 16, which may be a conventional MOSFET power transistor. The drain of the transistor 16 is connected to −VDC and its source is connected to a generator field winding 18 at its first end 20. A second end 24 of the generator field winding 18 is connected to +VDC. A free-wheeling diode 22 has its anode connected to the first end 20 of the generator field winding 18 and its cathode connected to +VDC.
In response to the voltage level of VPOR, the voltage regulator controller 12 provides a signal to the PWM controller 14, which triggers the PWM transistor 16. PWM transistor 16 is used to chop the DC voltage (VDC), thereby controlling the field voltage and consequently the field current IFIELD through the generator field winding 18. For example, during a load removal condition, the voltage regulator controller 12 causes the PWM controller 14 to reduce the field current. During this process, the field current keeps circulating through the free-wheeling diode 22. Unfortunately, this field current may not reduce quickly the energy to control the over-voltage condition and avoid damage to electrical devices attached to the generator.
As can be seen, there is a need for a voltage regulator circuit for electrical power generators which can rapidly respond to load removal and other fault situations to rapidly prevent over-voltage conditions at the generator terminal.